The present invention relates to a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus and, more particularly, to a device for transferring a toner image transferred from an image carrier to an intermediate transfer body further to a paper sheet or similar recording medium.
It is a common practice with a color image forming apparatus, which belongs to a family of image forming apparatuses of the kind described, to form toner images of three primary colors derived from a subtractive mixture on a sheet or similar recording medium one above the other. Specifically, the toner images are sequentially formed on a photoconductive element, once transferred to an intermediate transfer body (primary transfer) one above the other, and then collectively transferred to a sheet (secondary transfer).
An image transferring device of the type effecting the secondary transfer is disclosed in, e.g., Japanese Patent Laid-Open Publication No. 50170/1990. In this type of device, an intermediate transfer body is implemented as a belt having a specific surface resistance of 107 xcexa9/cm2 to 1012 xcexa9/cm2 and a specific volume resistance of 107 xcexa9cm to 1012 xcexa9cm. An electric field for causing the secondary transfer from the intermediate transfer body to occur is generated by an electrode implemented by a transfer roller. The transfer roller has its surface covered with a dielectric layer. A back electrode is located at the rear of the intermediate transfer body and faces the transfer roller to define a conduction path therebetween. A current flowing on the conduction path generates part of the electric field for the secondary transfer, thereby allowing a relatively low transfer bias to suffice.
However, the problem with the above-described configuration is that the resistance of the conduction path, as measured on the intermediate transfer body, is determined by the positional relation between the back electrode and the transfer roller. Specifically, in order that the current on the conduction path may be stabilized to reduce the required bias voltage for image transfer, the resistance between the back electrode and the transfer roller is required to be extremely low. It follows that the distance between the back electrode and the transfer roller has to be extremely short, i.e., several millimeters to 20 millimeters. This not only restricts the layout of the device but also requires extremely high positioning accuracy.
On the other hand, an implementation for preventing a member that faces the intermediate transfer body from being sequentially charged due to repetitive image formation is disclosed in, e.g., Japanese Patent Laid-Open Publication No. 288879/1989. According to this implementation, a conductive member in the form of a brush or sponge is held in contact with the member which faces the intermediate transfer body, thereby discharging the member. This, in principle, successfully discharges the facing member and, therefore, prevents the transfer characteristic from changing despite aging. In practice, however, it is difficult to carry out this scheme since the charge of the facing member is irregularly distributed and since the contact of the conductive member with the facing member cannot be easily set or maintained uniform. Although a charger may be used for the above purpose, it requires a high-tension power source which adds to the cost. Moreover, a charger produces ozone and nitrogen oxides which would deteriorate not only the facing member but also the intermediate transfer body itself.
It is, therefore, an object of the present invention to provide an image transferring device for an image forming apparatus which insures a stable electric field for image transfer without regard to the position of a back electrode.
It is another object of the present invention to provide an image transferring device for an image forming apparatus which insures a stable transfer characteristic by eliminating problems ascribable to the charging of a member that faces an intermediate transfer body.
An image transferring device incorporated in an image forming apparatus for transferring a toner image from a toner image carrier to a sheet of the present invention comprises an electric field forming member for forming an electric field for image transfer, and an electrode member connected to ground and located at the back of a portion of the toner image carrier which faces the electric field forming member. The electrode and located at the back of a portion of the toner fed from the electric field forming member to flow to ground and prevents a charge from being injected into the toner image.